1. Technical Field of the Invention
This invention relates to an improved lateral polysilicon diode and a method for forming same in an integrated circuit device.
2. Background Art
In general, the fabrication of a lateral diode in a film or layer of polysilicon, i.e., polycrystalline silicon, is well known to those skilled in the art. However, such diodes usually have high reverse current leakage if the polysilicon is doped sufficiently to lower the series resistance to provide satisfactory current flow before the diode saturates. The effects of dopant concentration on both forward current-voltage characteristics as well as reverse current leakage was discussed by Manoliu et al in "P-N Junctions in Polycrystalline-Silicon Films", Solid State Electronics, Volume 15, 1972, at pages 1103-1106.
Dutoit et al in "Lateral Polysilicon p-n Diodes" published in the Journal of Electrochem. Society: Solid-State Science and Technology, Volume 125, Number 10, October, 1978, at pages 1648-1651, stated that high value resistors or leaky diodes required in integrated circuits could easily be implemented using lateral polysilicon diodes. Dutoit et al noted that an excess reverse current, not accounted for by classical theories, was observed in heavily doped diodes.
While it is known that lowering the amount of doping will, in turn, lower the reverse current leakage of such polysilicon diodes, this has the undesirable effect of raising the series resistance of the diode which will cause it to saturate at too low a voltage resulting in low current flow.
Mandurah et al in "A Model for Conduction in Polycrystalline Silicon - Part I: Theory", in the IEEE Transactions on Electron Devices, Volume ED-28, Number 10, October, 1981, at pages 1163-1171, discussed some of the theories or models used to explain the resistivity variations with doping concentrations in polycrystalline silicon. Both the carrier-trapping model and the dopant-segregation model were discussed by the authors who proposed that the conduction may be a combining of the mechanisms of dopant-segregation, carrier-trapping and carrier-reflection at the grain boundaries of the polycrystalline silicon. Grain boundaries were assumed to behave as an intrinsic wide-band-semiconductor forming a heterojunction with the grains.
However, despite the amount of research and postulation as to the conduction phenomena occurring in P-N junctions formed from polycrystalline silicon, the fact still remains that if polysilicon is sufficiently doped to lower the series resistance to provide acceptable forward voltage drops, such high doping not only reduces the series resistance but also increases the reverse current leakage as well as reducing the breakdown voltage of the device.
It would, therefore, be desirable to fabricate a diode from polysilicon having reduced reverse current leakage while still maintaining a low enough series resistance to permit high forward current flow without saturating.